The present invention generally relates to optical lens assemblies and, more particularly, to lens assemblies used in projection televisions and other optical lens assemblies used in environments which are subject to variable temperatures.
A projection television set typically includes three cathode ray tubes (CRTs), corresponding to the primary colors, red, blue and green. A projection lens assembly uses a plurality lens to magnify the image appearing on the CRT faceplate and project that image onto a much larger viewing screen. CRTs used in projection televisions typically have a diameter of 3 to 9 inches. The image projected onto the screen generally has a size ranging from 40 to 60 inches or larger measured diagonally.
Each of the CRTs must provide maximum brightness or light intensity and, to facilitate this objective, each CRT operates at maximum power to produce maximum light output at the faceplate while still maintaining color balance. As a result, the CRTs generate considerable heat within the enclosure of the projection television set. It is not uncommon for portions of the projection television lens assemblies to be elevated by 40xc2x0 C. to 45xc2x0 C. or more above room temperature.
Each CRT has an associated magnifying lens system mounted adjacent to the CRT faceplate. Usually, the lens assembly is formed with at least one xe2x80x9cAxe2x80x9d lens element, at least one xe2x80x9cBxe2x80x9d lens element and at least one xe2x80x9cCxe2x80x9d lens element. Regardless of the number of lens elements, these are generally referred to in the art as xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d lens groups. That is, each xe2x80x9cgroupxe2x80x9d may be comprised of one or more lens elements. The xe2x80x9cBxe2x80x9d lens group usually includes a lens formed of glass, while the xe2x80x9cAxe2x80x9d and xe2x80x9cCxe2x80x9d lens groups may be formed of plastic. However, it should be understood that each group may comprise one or more lenses formed of glass and one or more lenses formed of plastic. Alternatively, the lens assemblies may comprise all glass lenses or all plastic lenses. Due to the heat generated by the CRTs, the plastic lenses will distort or otherwise undergo changes in optical properties. This is particularly true of the xe2x80x9cCxe2x80x9d lens which is mounted closest to the associated CRT. Also, the refractive index of liquid such as ethylene glycol used to cool the lens assemblies will also change due to temperature changes. Due to the temperature induced optical property changes such as these, the focus of the lens system can change. More specifically, as the temperature of the lens assembly changes over several hours of continuous operation, the picture displayed on the television screen could become blurred as a result of the defocusing effect of the increased temperature. This blurring effect can be more noticeable on high definition television (HDTV) sets due to their higher resolution capability.
Some temperature correction systems rely on one or more temperature sensors within a projection television cabinet and an automatic focusing mechanism which refocuses the lens assemblies based on feedback from the temperature sensor(s). Drawbacks to such systems include the expense and the relative difficulty in ensuring that each lens assembly is corrected in an effective and independent manner. One simpler manner of addressing the problems of heat-induced lens distortion is described in U.S. Pat. No. 6,104,554, assigned to the assignee of the present invention, and the disclosure of which is hereby fully incorporated by reference herein. The preferred embodiment shown and described in U.S. Pat. No. 6,104,554 utilizes a plurality of bars which thermally expand as the lens assembly is heated by the associated CRT. The bars are mechanically coupled to a lens cell carrying the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d lens elements. A xe2x80x9cCxe2x80x9d lens element is positioned closely adjacent to the CRT and distorts away from the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d lens elements. The bars undergo a similar heat induced distortion or expansion and thereby move the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d lens elements correspondingly in a direction toward the xe2x80x9cCxe2x80x9d element. This maintains proper spacing between the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d elements and the xe2x80x9cCxe2x80x9d element as the temperature in the interior of the television set increases. Likewise, when the television set cools down, the xe2x80x9cCxe2x80x9d element and the bars will return or retract to their original positions to maintain proper focus when the television set is turned on again.
It will be appreciated that such temperature induced defocussing can occur in various embodiments, including those which are subject to variations in temperature in either direction. Despite the improvements made in this area, there is a continuing need for lens assemblies which address the effect of temperature induced focusing problems while, for example, reducing the cost and complexity associated with manufacturing the lens assemblies.
Generally, the present invention provides a lens assembly adapted to be mounted in an environment subject to temperature change and providing for at least partial focus correction through an automatic relative movement of a direct or indirect lens mounting component by thermal expansion. A lens mount carries at least one lens and is coupled to a focus mount. Adjustment and locking structure is provided for allowing the lens mount to be axially adjusted relative to the focus mount and then locked into position. The lens mount and focus mount are formed of materials having different coefficients of thermal expansion (CTEs). Depending on the application, the material forming the focus mount can have a CTE which is lower than the CTE of the material forming the lens mount or, in other applications, the reverse situation may apply. In each case, when the lens mount and focus mount are subject to a temperature change, for example, when heated by a light source or when subjected to temperature change by some other environmental condition, the axial position of the lens mount will automatically change relative to the focus mount to move the lens axially after the two mounts are locked in position with the adjustment and locking structure. The automatic axial movement may be toward another lens or optical element or away from another lens or optical element depending on the application requirements. Automatic corrective movement will also take place through contraction in an environment that relatively changes from a heated state to a cooled state.
More specifically relative to the projection television industry, in a first embodiment xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d lens elements are carried by the lens mount and are axially adjusted and then mechanically fixed relative to a xe2x80x9cCxe2x80x9d lens element. At elevated temperatures, the lens mount will expand at a higher rate than the focus mount to move at least the xe2x80x9cAxe2x80x9d lens element toward the xe2x80x9cCxe2x80x9d element. This ensures that the proper lens spacing is maintained, as set during the initial adjustment, as the optical properties of the xe2x80x9cCxe2x80x9d element change in the heated environment.
In this embodiment, the focus mount and lens mount are cylindrical members and a mechanical fastener assembly is provided between these two members to allow the focus mount to be rotationally and axially adjusted relative to the focus mount. Once this manual adjustment is made, the focus mount is fixed relative to the lens mount and, in use, the automatic adjustment provided by the different CTEs of the two mounts takes over and automatically compensates for heat induced changes in optical properties. The manual adjustment is provided by a slot and threaded fastener assembly located approximately midway along the length of the focus mount and lens mount assembly. When fixed, this fastener assembly therefore provides a location from which the lens mount expands in opposite directions. The greatest expansion takes place toward the CRT since the temperature is highest in this region. Thus, at least the xe2x80x9cBxe2x80x9d lens group is mounted in the region of the lens mount positioned between the fastener assembly and the xe2x80x9cCxe2x80x9d element and CRT. Alternatively, both the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d lens groups may be positioned in this region such that they move essentially in unison toward the xe2x80x9cCxe2x80x9d lens group.
The preferred material for the focus mount in the first embodiment is a 10% glass filled polycarbonate having a coefficient of thermal expansion of 3.22xc3x9710xe2x88x925 cm/cm/C. The preferred material for the lens mount is unfilled polycarbonate having a coefficient of thermal expansion of 6.75xc3x9710xe2x88x925cm/cm/C. It will, however, be understood that other materials having different CTEs may be substituted while still falling within the spirit and scope of this invention. For example, although two plastic materials are used in this embodiment, metals may be substituted for one or both of these materials while achieving similar results.
In a second embodiment, the lens mount is formed of aluminum, while the focus mount is formed of plastic. In this embodiment the automatic corrective movement causes the lens or lenses carried by the lens mount to be moved away from another light transmissive or light generating component.
Additional objectives, advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings.